The present invention relates to a semiconductor fabrication technology, and more particularly, to an image sensor having a pad for electrically connecting a light-receiving element to an external line, and a manufacturing method for the same.
In recent years, a demand for a digital camera is explosively increasing with the development of visual communication technology using Internet. In addition, with an increasing spread of a mobile communication terminal such as a personal digital assistant (PDA), an international mobile telecommunications-2000 (IMT-2000), a code division multiple access (CDMA) terminal or the like, in which a camera is mounted, a demand for a miniaturized camera module is also increasing accordingly.
The camera module includes an image sensor. In general, a charge coupled device (CCD) and a complementary metal oxide semiconductor (CMOS) image sensor are widely used as the image sensor module. In the image sensor, to realize a color image, a color filter is aligned over a photo-detector that receives light from an exterior to generate photo-charges and accumulates the generated photo-charges. The color filter array (CFA) is configured with three color filters, e.g., red R, green G and blue B, or yellow, magenta and cyan. Typically, the three color filters with the red R, green G and blue B are mostly used as the color filter array in the CMOS image sensor.
Such an image sensor is a semiconductor device that converts an optical image into an electrical signal. As described above, the CCD and CMOS image sensor have been developed and are widely commercialized until now. The CCD has a structure in which each MOS capacitor is closely located and charges are stored in the MOS capacitor and transferred to a target. On the other hand, the CMOS image sensor employs CMOS technology that uses a control circuit and a signal processing circuit as peripheral circuits and adopts a switching mode that detects outputs sequentially. Here, MOS transistors are formed in the peripheral circuit as many as the pixels in the CMOS image sensor.
The CCD image sensor, however, has several disadvantages. Power consumption is too high. A manufacturing process is too complicated because of a plurality of masking processes. The CCD image sensor is hardly manufactured in one chip because it is difficult to embody a signal processing circuit within one chip. Thus, many researchers have attempted to develop such a CMOS image sensor using a sub-micron CMOS fabrication technology to overcome the above-listed disadvantages.
In the CMOS image sensor, a photodiode and MOS transistors are formed in a unit pixel, and thus signals are detected in sequence by a switching mode. As a result, an image can be implemented. Since the CMOS image sensor employs the CMOS fabrication technology, it has several advantages as compared with the CCD. Specifically, the power consumption of the CMOS image is lower than that of the CCD. In addition, the CMOS image sensor has a simplified fabrication process because it requires only about 20 masking processes whereas the CCD requires 30 to 40 masking processes. Further, various signal processing circuits and others can be integrated within one chip. For these reasons, the CMOS image sensor is being highlighted as a next generation image sensor.
Generally, the CMOS image sensor includes a photo-detector for detecting light and a logic circuit component for processing the detected light into an electrical signal, which is, in turn, systemized into data. There have been numerous attempts to improve a fill factor, which represents an area ratio of the light-sensing element with respect to the overall image sensor. However, these attempts are limited since the logic circuit component cannot be basically removed. Therefore, there is introduced a light condensing technology for changing paths of incident lights that enter other areas except the light-sensing element and condensing the incident lights into the light-sensing element so as to enhance photosensitivity. To realize the light condensing technology, a method for forming a microlens on the color filter of the image sensor is particularly used.
FIGS. 1A to 1F are cross-sectional views illustrating a method for manufacturing a typical CMOS image sensor. Herein, the region A denotes a region where a pad will be formed (hereinafter, referred to as the pad region A), and the region B denotes a region where a light-receiving region where a light-receiving element of a unit pixel will be formed (hereinafter, referred to as the light-receiving region B).
Referring to FIG. 1A, an aluminum (Al) metal interconnection 101 is formed over an underlying structure having various elements such as a photodiode and transistors constituting an image sensor. A nitride layer 102, which acts as an etch stop layer during an etching process for forming a pad contact, is formed over the Al metal interconnection 101.
An oxide layer 103 and a nitride layer 104, which act as a protective layer for protecting the Al metal interconnection 101, are formed along a surface profile of the resultant structure including the Al metal interconnection 101. A photoresist pattern 105 is formed over the nitride layer 104 for forming a pad contact, wherein the photoresist pattern 105 exposes a portion of the nitride layer 104 corresponding to the Al metal interconnection 101 of the pad region A.
Referring to FIG. 1B, the nitride layer 104 is etched through an etching process using the photoresist pattern 105 as an etch mask to define a region 106 where a pad will be formed. At this time, the oxide layer 103 is also etched partially. Here, reference symbols 104A and 103A denote a remaining nitride layer and a remaining oxide layer, respectively, after the etching process. Afterwards, the photoresist pattern 105 is removed.
Referring to FIG. 1C, a lower over coating layer (OCL) 107, color filters 108, an upper OCL 109, and microlenses 110 are sequentially formed over the light-receiving region B. Referring to FIG. 1D, a low temperature oxide (LTO) layer 111 is formed along a surface profile of the resultant structure including the microlenses 110.
Referring to FIG. 1E, a photoresist pattern 112 is formed over the LTO layer 111 for forming a pad contact, wherein the photoresist pattern 112 exposes a portion of the LTO layer 111 corresponding to the Al metal interconnection 101 of the pad region A.
Referring to FIG. 1F, the exposed portion of the LTO layer 111, the remaining oxide layer 103A, and the nitride layer 102 are sequentially etched using the photoresist pattern 112 as an etch mask, thereby forming an opening 113 exposing the Al metal interconnection 101. Here, reference symbols 111A, 103B and 102A denote an LTO pattern, an oxide pattern, and a nitride pattern, respectively. Thereafter, the photoresist pattern 112 is removed.
As described above, in the typical method for manufacturing the image sensor, the LTO layer is formed for protecting the light-receiving element. The light-receiving element plays a role in condensing light incident from an exterior onto a photodiode, and is configured with color filters and microlenses. Thus, it is necessary to form a protective layer for protecting the light-receiving element physically or chemically. The reason is that there always exist external environmental factors such as moisture, particles and thermal stress in a fabrication process line, which may damage the color filters and microlenses, even after forming the color filters and the microlenses. However, according to the typical method for manufacturing the image sensor using the LTO layer as a final protective layer, a peeling phenomenon in which a portion of the LTO layer is peeled off during the fabrication process may occur.
FIGS. 2A and 2B illustrate cross-sectional views showing peeling and crack phenomena of the LTO layer.
After forming the opening (113 of FIG. 1F), an ashing process is performed to remove the photoresist pattern (112 of FIG. 1E). However, referring to FIG. 2A, during the ashing process, O2 plasma gas often flows into the upper OCL 109 at a portion marked as a circular dotted line 130, which causes anisotropic etching on the upper OCL 109.
Thus, referring to FIG. 2B, a space is created between the remaining nitride layer 104A acting as the passivation layer and the LTO layer 111. This space leads to the peeling phenomenon in which a portion of the LTO layer 111 is peeled off during a subsequent fabrication process and package process. Moreover, when a crack exists in the LTO layer 111 due to the peeling phenomenon, the microlens is likely to be damaged due to O2 plasma. This damage is illustrated as a circular solid line in FIG. 3, and this damage may result in a process failure.